The simple answer is No. No, you will not be able to plug in your hard drive into the NAS, and still see all your current Windows data.

You MUST format the disk first, before you can use it with the NAS. So, you will have to find a place to offload all your data to, then format the NAS hard drive you want to use, and then copy all the data back over.

Like 'u3gyxap' said , if you've activated telnet, you will be able to mount anything you want.
!!! BUT !!! , that's a really bad solution , you'll certainly loose files/data.
fat/ntfs libraries aren't included in the firmware, so handling such partitions isn't possible without problems.

so i second 'swambast' to say , YOU MUST FORMAT it first with the nslu2 utility.

(i did it and see, after few weeks , that many files were 0 bytes !! that can be really irritating with important work in progress)